My invention relates to tubes of non woven material provided with supporting membranes for membrane filtration on its inner wall, said tube consisting of a band of a sealable synthetic fibres containing non woven material which is wound in a helical or non helical way, consecutive windings of the band being interconnected by heatsealing. By "membrane filtration" is meant e.g. ultrafiltration and reversed osmosis.
Such tubes of non woven material in which is disposed a tubular membrane for purifying liquids e.g. by means of reversed osmosis, are known in the art. They are e.g. manufactured by helically winding and with an overlap, a polyester non woven on a mandril, while between the overlapping parts of the band a polyethylene glue is applied which is heated to a temperature ranging from 110.degree. to 150.degree. C., whereupon the further sealing is effected by means of a heated shoe.
Due to the presence of the overlaps it is observed that within the tube sharp and/or fibrous edges are found whereby when e.g. a membrane suitable for reversed osmosis is directly applied to the non woven these edges show themselves in the membrane and give rise to weakenings in the membrane. Moreover there is a risk of projecting fibre ends of the non woven protruding through the membrane.
In order to obviate these difficulties it is known in the art to wind along a paper sheet on the inside of the non woven tube during the winding operation. In this way the aforementioned drawbacks of the application of tubes of non woven material obtained by winding a non woven of thermoplastic fibres in an overlapping way, are eliminated, but paper suffers from the disadvantage that, when softened in water, in the long run the coherence gets lost for the greater part, while the slimy mass thus produced, when the material is confined in a narrow space and when a pressure is exerted, offers a considerable resistance to the passage of water. Especially this unnecessary resistance to the passage of water should be avoided since it gives rise to waste of energy.